This invention is directed to apparatus for vacuum molding of thermoplastic workpieces, and is more particularly directed to rotary thermoformers in which a plastic workpiece is loaded to a frame at a loading station, and carried to a heating station, and then to a molding station, and then to an unloading station where the finished molded workpiece is removed from the frame.
A standard rotary thermoformer has a carousel that contains three, four, or five standard frames disposed about a vertical rotary axis. Each of the frames has a front main frame member, a back main frame member, and left and right side frame members. These each carry a set of clamp frames. The clamp frames consist of front and back clamp frame members that are suspended at each end from the side frame members, and side clamp frame members that are supported between the front and back clamp frame members. The front and back clamp frame member positions are adjustable, as are the positions of the side frame members, so that the clamp frame dimensions can match the workpiece dimensions. The side clamp frame members can have a telescoping construction so that their lengths match the spacing between the front and back clamp frame members. Pneumatic clamps and pinbars are installed on the clamp frame to hold the workpiece in place as it is carried from station to station.
For a single mold at or close to the size of the frame capacity, the standard front, back and side clamps would be used as is, and only a single mold could be employed. However, if two or more smaller molds are to be run simultaneously it is possible to run two mold jobs at once on the same thermoformer, but only if a number of constraints are satisfied.
With this standard system, there is very little flexibility in accommodating more than one molding job at the same time. A standard double mold set up can be accomplished in two ways, both of which require that the workpieces have a common dimension (i.e. length or width).
If sheet factors permit (i.e., sheet type, sheet color, sheet gauge, direction of extrusion, etc.) and if the work pieces have one common dimension, then the molds can be butted together and one larger sheet can be run to cover both molds. In this situation, a standard single-frame set up would suffice. However, an extra mold step, that is, cutting apart the two molded workpieces, is required.
A second method can be used if the sheet factors are different, as long as the molds still have one common dimension. In this method the standard front and back clamp frames are used, but there are two sets of side clamp frames for each step.
If a smaller-dimension mold is used with a larger one, this second method could be used, but a larger workpiece would be needed for the smaller mold. This requires a trimming step to trim the excess plastic sheet after molding. The extra labor and the plastic waste factor can make this procedure economically unfeasible.
The plastics molding industry has long sought a straight-forward, reliable, and economic procedure to maximize the capacity of rotary thermoformers, but to date this goal has proved elusive.